They need careful cleaning. Put a deep cut from top of leek down almost to the base. Wash leaf by leaf under running water.
If you don't you may be like a friend of mine (poor cook) who said she liked the leeks, but not the crunchiness. The crunchiness was the sand the leeks grew in. You need to wash these very meticulously, esp. right to the base. Also there is a kind of pocket which will catch the dirt on the leaves which will require special attention when they are washed. Try Vichyssoise. Google it for a recipe.
It is a base of potatoes, fresh leeks, milk, cream, chicken stock and seasonings. Beautiful - often served cold in summer but also good hot during winter.
